'Light trap' is a step towards quantum memory

The best place to hold the nebulous stuff called quantum information might be within an appropriately nebulous container - a small, cold cloud of gas.

Two teams have independently succeeded in placing a cloud of chilled rubidium atoms within an optical cavity, which traps light between two opposed mirrors. The combination could one day form a quantum memory element.

Quantum information deals not in the ordinary bits of classical information (0 or 1, yes or no) but in qubits, which can be a blend of 0 and 1, yes and no. Using qubits, physicists have devised uncrackable codes and hope to build ultra- fast quantum computers.

In theory, you can store a qubit by using a photon to change the energy levels of an atom. The photon's mixed yes-and-no quantum state is written onto the atom, which ends up in the same mixture of high- and low-energy states.

Reading this type of memory can be treacherous, however. The atom could simply drift away, or it could re-emit a photon containing the qubit in a random direction. Either way, the memory can easily be lost.

Super-atom

These problems might be solved by using not one atom but a collection of them, in a state called a Bose-Einstein condensate, or BEC. All the atoms in a BEC share the same quantum state. "They act collectively, like a 'super-atom'," says Yves Colombe of the Kastler Brossel Laboratory in Paris, France.

Colombe, Jakob Reichel and colleagues have put a BEC between two mirrors, which form an optical cavity trapping photons of a particular wavelength. They have shown that the BEC can be forced to respond to exactly that wavelength, so that it should only emit photons in a controlled direction within the cavity.

A BEC can more easily be cooled to very low temperatures than a single atom, says Tilman Esslinger of the Institute for Quantum Electronics in Zurich, Switzerland, who leads the second team. The cold BEC has no thermal motion and does not drift away, meaning the information held in it could be stored for longer.

Building the cavity was a particularly tricky technical problem for Colombe's team, as their aim was to fit it onto a single chip. They developed a new type of cavity, in which the mirrors are painted onto the ends of two optical fibres just 0.04 millimetres across. They used a laser to evaporate material from each fibre, sculpting an extremely smooth mirror surface.

The new experiments show that a BEC can be plugged into an optical cavity, but Esslinger is keen to point out they cannot yet function as quantum memory elements. For now, he says, the objective is simply to learn how this new combined system behaves.

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